Most scientists think the dinosaurs — along with countless other creatures — were wiped out some 66 million years ago when a space rock slammed into Earth. But a cosmic impact isn’t the only disaster that could have rapidly extinguished a huge percentage of life on Earth; nearby supernovae can pose a similar risk.
If a supernova erupted near Earth, harmful cosmic rays, which are charged particles that act like tiny space bullets, and ultraviolet radiation would pummel Earth’s ozone layer, eventually tearing a hole through its protective bubble. This scenario has long been considered, but a recent study
published in the Proceedings of the National Academy of Sciences suggests that at least one of Earth’s past mass extinctions might have been the result of a nearby supernova.
The crime scene
The Devonian period, which lasted from about 419 million to 359 million years ago, culminated in a mass extinction event. During the Devonian, life on dry land was just getting started, and plants led the way. Meanwhile, marine life was already substantially diverse, earning the period the nickname Age of Fishes.
By examining the boundary between the Devonian period and the next (Carboniferous Period) in Earth’s soil, the scientists behind this latest study found plant spores that were burned by ultraviolet light, which suggests they were around when the ozone was depleted. And while numerous calamities can take chunks out of Earth’s ozone, the researchers argue that in this case, a supernova is the most likely culprit.
“Earth-based catastrophes such as large-scale volcanism and global warming can destroy the ozone layer, too,” lead author Brian Fields
said in a statement. But the evidence for those scenarios just isn’t there. “Instead, we propose that one or more supernova explosions, about 65 light-years away from Earth, could have been responsible for the protracted loss of ozone.”
A supernova could indeed deliver just the right attack to devastate life on Earth. Such an explosion would bathe Earth in harmful ultraviolet light, but the damage wouldn’t end there: For up to 100,000 years, supernova debris would continue to rain down on our planet, creating radioactive isotopes in Earth’s atmosphere.
But to complicate matters further, fossil evidence indicates that biodiversity fell for a total of
300,000 years before the final Devonian mass extinction occurred. This suggests that there might have even been more than one supernova that thrashed Earth. “This is entirely possible,” said study co-author Jesse Miller. “Massive stars usually occur in clusters with other massive stars, and other supernovae are likely to occur soon after the first explosion.”
Although the team is still missing conclusive evidence to confirm their theory, they have already outlined what they need to look for. The radioactive isotopes created by supernova debris interacting with Earth’s atmosphere would have decayed long ago. So if researchers can find evidence of these isotopes in rocks from the Devonian-Carboniferous boundary, it would directly suggest that at least one nearby star exploded around that time.
